1. Field of the Invention
This invention relates to a bit rate reduction and, more particularly, to data compression and expansion methods and apparatus for processing digital characters.
2. Description of the Prior Art
In a pulse code modulation (PCM) system, a typical analog-to-digital encoder functions to quantize an analog sample and to generate a digital character. In nonuniform PCM encoding, for example, encoding according to a .mu. = 255 companding law such as is utilized in the D2 and D3 channel banks of the Bell System, each analog sample is converted into an eight-bit digital character. The initial bit of the digital character represents the sign or polarity of the analog sample while the coarse amplitude of the analog sample is represented by the three bits following the sign bit. Each value of these three bits, knows as a segment or chord value, represents one of eight amplitude ranges. The value of consecutive segments corresponds to a doubling in magnitude of the analog sample. Finally, the remaining four digital bits, called the mantissa bits, represent one of sixteen usually equal length intervals present in each one of the segments or chords.
In copending application D. L. Duttweiler-D. G. Messerschmitt, Ser. No. 517,879, filed Oct. 25, 1974, a digital block processor is disclosed for compressing a predetermined number of nonuniformly encoded PCM digital characters, called a PCM block. The PCM block is stored in a digital delay while the maximum segment value present in the block is determined. The three-bit maximum value is then transmitted to a receiving station. Thereafter, character-by-character, each PCM character in the block is advantageously coded to produce a character having fewer bits, called a Nearly Instantaneous Companded (NIC) digital character. The plurality of NIC characters is called a NIC block. Each illustrative NIC character in a NIC block consists of six bits. The initial bit is the same as the sign bit as in the PCM character. The second bit is determined as follows: (1) If the PCM character has a segment value equal to the maximum segment value and the maximum value is not "000", the three bits representing the segment value are simply replaced by a logic one so that the second bit in the NIC character becomes a logic one, or (2) if either the PCM character has a segment value not equal to the maximum segment value or the maximum segment value is "000", the second bit in the NIC character becomes a logic zero. The remaining four bits in the NIC character are obtained responsive to the following shift operation. The mantissa bits for a stored PCM character are placed in a shift register. The register is thereafter right shifted by an amount which is dependent on the difference between the maximum segment value and the segment value represented by the segment bits of the stored PCM character. The resultant 6-bit NIC character is then transmitted to the receiving station. At the receiving station, the maximum segment value and the NIC character are utilized to expand the NIC character so as to "reconstruct" a PCM character similar to the compressed PCM character. Of course, futher compression, expansion, compression, expansion et cetera is possible and is herein called tandem PCM-to-NIC-to-PCM processing.
In the prior art illustrative embodiment, eight 8-bit digital characters are assumed to be present in each PCM block. Accordingly, it is clear that without data compression a PCM block of 64 bits would be transmitted to the receiving station. In contradistinction, when the prior art 6-bit NIC data compression arrangement is employed, a NIC block plus maximum segment value of only 51 bits is transmitted, a decided reduction in bit rate and concomitant reduction in bandwidth, albeit not without some signal degradation.
Unfortunately, in tandem PCM-to-NIC-to-PCM processing, if the PCM block phase used upon a subsequent PCM-to-NIC compression is different than that used with its original PCM-to-NIC compression, still further signal degradation may be obtained. This unfortuitous result obtains because the incorrect maximum segment value may be used during the subsequent compression.
accordingly, a broad object of my invention is to provide improved data compression and expansion methods and apparatus for mitigating signal degradation in tandem digital processing.
Further in tandem processing, a signal delay accumulation problem may arise. Specifically, this problem arises in known prior art arrangements because the maximum segment value is unknown until the plurality of PCM characters which are to be compressed to make up the NIC block have been received.
Accordingly, another broad object of my invention is to provide improved data compression and expansion methods and apparatus for mitigating signal delay.